I propose to isolate animal tissue culture cells with alterations in their ability to make or regulate ornithine decarboxylase (ODC). This enzyme makes putrescine, the structurally simplest polyamine. The enzyme can be made to undergo very large changes in activity in multiple systems in response to a variety of manipulations. Often, conditions that result in increased growth lead also to an increase in ODC activity. The mechanisms responsible for determining the activity of this enzyme and the size of polyamine pools remain to be clarified. They may be significant for understanding normal cell proliferation and malignant transformation. I propose a novel approach to this problem that consists of isolating and studying mutant cells with specific defects in polyamine metabolism, an approach that has already proven valuble in micro-organisms. I will use for this purpose S49 mouse lymphoma cells and CHO Chinese hamster ovary cells. Mutants will be selected that are absolutely deficient in ODC activity or whose deficiency is conditional on temperature of cell growth. An attempt will also be made to isolate mutants that are altered in their regulation of ODC activity by the cyclic nucleotides or by polyamines. These mutants will be characterized to determine the specific nature of their defect. They will be used to study the structure, regulation, and function of ODC in these cells. By use of the mutants it will be possible to determine the effects of varying polyamine pool size and composition on intracellular RNA, protein, and DNA synthesis and on cell growth.